1. Field of the Invention
The present invention relates generally to laminated common-mode choke coils and, more particularly, to a structure of a laminated common-mode choke coil in which the absolute value of normal-mode impedance can be reduced.
2. Description of the Related Art
As one type of common-mode choke coil, a laminated common-mode choke coil is available having a structure in which thin metallic patterns in the form of a coil are interposed between magnetic substrates. Two examples of this type of choke coil are disclosed in, for example, Japanese Unexamined Patent Publication No. 4-364709. The choke coil disclosed in this publication is constructed to have a plurality of layers in the following manner: a primary coil conductor and a secondary coil conductor are laminated through an insulating layer in the thickness direction according to a thin-film forming process, such as photolithography. In this laminated structure, since the insulating layer is formed according to a thin-film forming process, the thickness of the resulting layer can be reduced. This makes it possible to decrease the distance between the primary coil conductor and the secondary coil conductor opposedly facing each other across the insulating layer. Thus, a high coupling coefficient can be obtained, and a high impedance in response to common-mode noise can be achieved. As a consequence, a laminated common-mode choke coil exhibiting excellent performance in eliminating common-mode noise can be obtained.
However, the choke coil disclosed and shown in FIG. 5 in the above publication disadvantageously changes the waveform of normal-mode signals. This arises from a disturbance in magnetic balance between the primary and secondary coil conductors caused by a difference in the number of turns between the conductors.
In order to solve this problem, in the laminated common-mode choke coil disclosed and illustrated in FIG. 1 in the above publication, the configuration of the coil is adjusted to have the same number of turns between the primary and secondary coil conductors. The outer appearance of such a choke coil is shown in FIG. 5. In this choke coil generally indicated by 50, the number of turns between the primary and secondary coil conductors is made equal so that external electrodes 51 and 52 used as input/output electrodes for the primary coil conductor are diagonally disposed on the chip, and so that external electrodes 53 and 54 serving as input/output electrodes for the secondary coil conductor are also diagonally placed on the chip. With this arrangement, however, when such a coil is mounted for use, it is necessary that wiring to be connected to the primary coil conductor and to the secondary coil conductor be diagonally laid, thereby resulting in a complicated wiring pattern for mounting the coil.
To further overcome the above drawback, the following type of choke coil has been invented: a laminated common-mode choke coil having the same number of turns for the primary and secondary coil electrodes in which the external electrodes for the primary coil electrode are aligned on the same side of the chip and the external electrodes for the secondary coil electrode are also aligned on the opposing side of the chip. FIG. 6 is an exploded perspective view of an example of known laminated common-mode choke coils having the above-described construction; FIG. 7 is a plan view illustrating the pattern configuration of the primary and secondary coil electrodes; and FIG. 8 is a cross sectional view taken along line Yxe2x80x94Y of FIG. 6.
Referring to FIGS. 6 through 8, a laminated common-mode choke coil generally designated by 10 is constructed by laminating a plurality of insulating layers 13A, 13 and 15 between a pair of magnetic substrates 11 and 17. Formed between the insulating layers 13A and 13 is a primary coil electrode 12a formed of a thin metallic film. The primary coil electrode 12a is connected at one end to an external electrode 3a and at the other end to a node 22a between the electrode 12a and first through-hole electrodes 23a and 25a. Also, a secondary coil electrode 14b is disposed between the two insulating layers 13 and 15. The electrode 14b is connected at one end to an external electrode 2b and at the other end to a node 24b between the electrode 14b and a second through-hole electrode 25b. 
Further, formed between the insulating layer 15 and magnetic substrate 17 are a lead electrode 16a for the primary coil electrode 12a and a lead electrode 16b for the secondary coil electrode 14b. The primary-coil lead electrode 16a is connected between an external electrode 4a and a node 26a between the electrode 16a and the first through-hole electrodes 23a and 25a, while the secondary-coil lead electrode 16b is connected between an external electrode 5b and a node 26b between the electrode 16b and the second through-hole electrode 25b. 
In this common-mode choke coil 10, the number of turns of the primary coil electrode 12a and that of the secondary coil electrode 14b are set substantially equal to each other, and more particularly, the former is set to approximately 2T (turns), while the latter is set to 2xc2x71/8T. Additionally, the external electrodes 3a and 4a for the primary coil electrode 12a are disposed to project from the same side of the rectangular-prism-shaped chip, and the external electrodes 2b and 5b for the secondary coil electrode 14b are also placed to project from the opposite side of the chip.
As described above, in the above type of choke coil, in order to keep the waveform of normal-mode signals from changing, it is important to maintain a magnetic balance between the primary and secondary coil electrodes. Accordingly, in the choke coil illustrated in FIGS. 6 through 8, the number of turns of the primary coil electrode 12a is set equal to that of the secondary coil electrode 14b. 
If the coil pattern is arranged so that the number of turns of the primary coil electrode 12a can be equal to that of the secondary coil electrode 14b, as illustrated in FIG. 7, there are disadvantageously created portions in which the primary and secondary coil electrodes 12a and 14b are not overlapped in the vicinity of the through-hole electrodes 23a and 25b. Magnetic coupling force between the coil electrodes 12a and 14b is weaker in such non-overlapping portions, thereby increasing the impedance in relation to normal-mode signals.
Accordingly, it is an object of the present invention to provide a laminated common-mode choke coil that can reduce the impedance in the normal mode by improving the magnetic coupling force between a pair of coil electrodes.
In order to achieve the above object, according to the present invention, there is provided a laminated common-mode choke coil comprising: a laminated structure having a plurality of layers including a pair of magnetic substrates and an insulating layer interposed between the pair of magnetic substrates; primary (first) and secondary (second) coil electrodes disposed on the different layers of the laminated structure; first through fourth external electrodes formed at the edges of the laminated structure, the first external electrode being connected to one end of the primary coil electrode, and the third external electrode being connected to one end of the secondary coil electrode; and first and second through-hole electrodes through which the second external electrode is connected to the other end of the primary coil electrode and the fourth external electrode is connected to the other end of the secondary coil electrode, wherein said primary and secondary coil electrodes are spiral in shape, and said primary coil electrode extending from a connecting portion with said first external electrode and overlapping with said secondary coil electrode except at a position between said first and second through-hole electrodes.
According to a more specific aspect of the present invention, there is provided a laminated common-mode choke coil comprising: a pair of first and second magnetic substrates; a rectangular-prism-shaped laminated structure having first and second insulating layers laminated between the first and second magnetic substrates; a primary coil electrode formed between the first magnetic substrate and the first insulating layer, and a first external electrode connected to one end of the primary coil electrode; a first lead electrode formed between the second insulating layer and the second magnetic substrate and electrically connected at one end to the other end of the primary coil electrode; a second external electrode connected to the other end of the first lead electrode; a secondary coil electrode formed between the first and second insulating layers, and a third external electrode connected to one end of the secondary coil electrode; a second lead electrode formed between the second insulating layer and the second magnetic substrate and electrically connected at one end to the other end of secondary coil electrode; a fourth external electrode connected to the other end of the second lead electrode; first through-hole electrodes formed inside the first and second insulating layers and connecting the other end of the primary coil electrode and one end of the first lead electrode; and a second through-hole electrode formed inside the second insulating layer and connecting the other end of the secondary coil electrode and one end of the second lead electrode. The first and second external electrodes are positioned to be partially exposed from a first lateral surface of the laminated structure, while the third and fourth external electrodes are positioned to be partially exposed from a second lateral surface opposing the first lateral surface of the laminated structure. The primary coil electrode is formed in a spiral shape in an area from the first external electrode to the first through-hole electrode, while the secondary coil electrode is formed in a spiral shape in an area from the third external electrode to the second through-hole electrode. Further, the primary coil electrode is spirally configured in such a manner that it extends from the first external electrode and overlaps with the secondary coil electrode until a position where the primary coil electrode reaches the second through-hole electrode.
According to a restricted aspect of the present invention, there is provided a laminated common-mode choke coil in which the primary coil electrode may be linearly formed from a position where it passes over the second through-hole electrode to a position where the primary coil electrode reaches the first through-hole electrode.
According to another aspect of the present invention, there is provided a laminated common-mode choke coil comprising: laminated structure having a plurality of layers including a pair of magnetic substrates and a first insulating layer interposed between said pair of magnetic substrates; primary and secondary coil electrodes disposed on the different layers of said laminated structure; first through fourth external electrodes located at edge of said laminated structure, said first external electrode being connected to one end of said primary coil electrode, and said third external electrode being connected to one end of said secondary coil electrode; and first and second through-hole electrodes through which said second external electrode is connected to the other end of said primary coil electrode and said fourth external electrode is connected to the other end of said secondary coil electrode.
In the laminated common-mode choke coil constructed in accordance with the present invention, since the primary and secondary coil electrodes are spirally configured as described above, the overlapped portion between the primary and secondary coil electrodes in the vicinity of the second through-hole electrode can be increased as compared with conventional choke coils having the spiral shape shown in FIG. 7. This improves the magnetic coupling force between both the coil electrodes, thereby reducing the impedance in relation to normal-mode signals.